In dentistry, practitioners use a variety of restorative materials in order to create crowns, veneers, direct fillings, inlays, onlays and splints. Composite resins are a type of restorative material which are suspensions of strengthening agents, such as mineral filler particles, in a polymerizable resin matrix. These materials may be dispersion reinforced or particulate reinforced, depending on the type of filler, or may be hybrid composites or flowable composites, depending on the filler loading. A full discussion of these materials is included in U.S. patent application Ser. No. 09/181,507, entitled "Optimum Particle Sized Hybrid Composite," C. Angeletakis, et al., filed on Oct. 28, 1998 now pending (incorporated herein by reference in its entirety). Highly pure submicron particles are useful in these composite resin materials because they impart the desirable optical properties of high gloss and high translucency.
While agitator ball mills are known for producing submicron particles, they have previously not been used to produce particles for filler in dental composites because of the impurities which result. The inclusion of impurities in dental composites can decrease translucency and negatively affect color. Prior art mills are set forth in U.S. Pat. Nos. 5,335,867; 4,129,261; and 4,117,981, all assigned to Draiswerke GmbH and each incorporated herein by reference in its entirety; and 5,065,946, assigned to Matsushita Electric Industrial Co. and incorporated herein by reference in its entirety. These prior art mills typically include ceramic or metallic agitators and grinding chambers. During milling, the ceramic or metallic material of the agitator and grinding chamber spalls and abrades, and the abraded particles become intimately mixed with the material being ground. In the case of fillers for dental restoratives, these abraded particles are unacceptable due to their impact on the optical properties of the restorative. The abraded particles may cause decreased translucency due to light scattering and may impart an unnatural color. Draiswerke, Inc., Mahwah, N.J., has applied a polyurethane coating on the agitator and grinding chamber for their PML-H/V machine. The pigment from this coating, however, also contaminates the composites, making them unacceptable for dental use.
The predominant types of milling methods are dry milling and wet milling. In dry milling, air or an inert gas is used to keep particles in suspension. However, fine particles tend to agglomerate in response to van der Waals forces which limits the capabilities of dry milling. Wet milling uses a liquid such as water or alcohol to control reagglomeration of fine particles. Therefore, wet milling is typically used for comminution of submicron-sized particles.
A wet mill typically includes spherical media that apply sufficient force to break particles that are suspended in a liquid medium. Milling devices are categorized by the method used to impart motion to the media. The motion imparted to wet ball mills includes tumbling, vibratory, planetary and agitation. While it is possible to form submicron particles with each of these types of mills, the agitation or agitator ball mill is typically most efficient.
The agitator ball mill, also known as an attrition or stirred mill, has several advantages including high energy efficiency, high solids handling, narrow size distribution of the product output, and the ability to produce homogeneous slurries. The major variables in using an agitator ball mill are agitator speed, suspension flow rate, residence time, slurry viscosity, solid size of the in-feed, milling media size and desired product size. As a general rule, agitator mills typically grind particles to a mean particle size approximately 1/1000 of the size of the milling media in the most efficient operation. In order to obtain mean particle sizes on the order of 0.05 .mu.m to 0.5 .mu.m, milling media having a size of less than 0.45 mm can be used. Milling media having diameters of about 0.2 mm and about 0.6 mm are available from Tosoh Ceramics, Bound Brook, N.J. Thus, to optimize milling, it is desirable to use a milling media approximately 1000 times the size of the desired particle. This minimizes the time required for milling.
Previously, the use of a milling process to achieve such fine particle sizes was difficult due to contamination of the slurry by the milling media. By using yttria stabilized zirconia (YTZ or Y-TZP, where TZP is tetragonal zirconia polycrystal) the contamination by spalling from the milling media and abrasion from the mill is minimized. Y-TZP has a fine grain, high strength and a high fracture toughness. High strength Y-TZP is formed by sintering at temperatures of about 1550.degree. C. to form tetragonal grains having 1-2 .mu.m tetragonal grains mixed with 4-8 .mu.m cubic grains and high strength (1000 MPa), high fracture toughness (8.5 MPa m.sup.1/2) and excellent wear resistance. The use of Y-TZP provides a suitable milling media for providing relatively pure structural fillers having mean particle sizes less than 0.5 .mu.m. The YTZ milling media, however, is very expensive. So although agitator milling with YTZ milling media is time efficient, it is costly due to the expense of the milling media as well as the cost of the machine.
Furthermore, despite some reduction in contamination of the ground filler particulate by the use of YTZ milling media, agitator ball mills still introduce an unacceptably high level of contamination into dental composites containing the ground filler. The high intensity of the grinding action produced by the agitator, and the high momentum of the media, result in abrasion and spalling of the grinding chamber wall.
Vibratory ball mills are often used for submicron particle grinding because they provide a high production rate at low capital cost, fine and uniform product size distribution, low power consumption, and low contamination. The rate of milling is a function of the shape and size of the media. Cylindrical media are generally preferred, according to Engineered Materials Handbook.RTM., Desk Edition, ASM International, p 742 (1995), because they spin on an axis and therefore produce small shear forces. The major variables in using a vibratory mill are the amplitude of vibration, energy developed in the mill, slurry viscosity, solid size of the in-feed, milling media size and desired product size. Because vibratory milling involves low intensity grinding, abrasion and spalling of the grinding chamber wall and the milling media are less of a concern, as compared to agitator mills.
The vibratory mill has not previously been found useful for low contamination grinding of particles having a size less than the wavelength of light. U.S. Pat. No. 4,544,359 describes a dental restorative material with a borosilicate glass/barium silicate filler having an average particle size diameter of from about 0.5 .mu.m to 5.0 .mu.m. The filler is ground by a conventional wet milling process, such as vibratory milling, using a grinding or milling media such as low alumina, porcelain balls, stainless steel balls, borosilicate glass rods, or any other low alumina, non-contaminating grinding medium. Thus, there is a need for a low contamination, vibratory grinding mill and method to produce particles having a mean particle size of less than 0.5 .mu.m.